This invention applies to the field of fiber optic light guides, and more particularly to a bundle of light guides held in an aperture and receiving light from a fiber optic projector.
A principal disadvantage of such currently known fiber optic light guides is that the intense illumination from a light projector causes the light guides to heat, age and often burn the surface of the fiber ends, destroying the light guide prematurely.
Glass optical fibers are very heat resistant, but the fiber ends are bundled together and potted with epoxy, making the entrance end of the light guide heat sensitive, as the epoxy ages and chars around the glass fibers at temperatures around 160.degree. C. Therefore, it is necessary to limit the temperature to the operating temperature of the epoxy, not the glass fibers.
Some plastic light guides have a soft, "solid core" construction, in which the flexible core is sheated in a thin-wall tube of heat-resistant plastic. The light-transmitting solid core can operated at temperatures as high as 150.degree. C., but in long-term use, the core ages, discolors, cross-links and embrittles.
Acrylic (polymethyl methacrylate) fibers are low cost light guides that have very accurate color transmission. This combination of low cost and true color transmission makes it very desirable to use acrylic light guides for commercial lighting applications. However, the maximum continuous operating temperature for acrylic optical fibers is only 70.degree. C. If operated at or below the continuous service temperature, acrylic fibers will last for more than a decade without aging or discoloring. However, above 80.degree. C. the acrylic plastic fiber ends begin to soften, distort and melt. Therefore, acrylic fiber manufacturers recommend that the light source not be focussed on the fiber bundle, so the light is not concentrated on the center of the fibers. This reduces melting and burning the fibers, but such defocussing throws much of the light outside the bundle, diminishing the optical efficiency of the system.
Since light guide temperature is critical for all types of fibers, many thermal control techniques are presently used to reduce the heat to protect the end of the fiber bundles. Thermal control techniques include the use of dichroic reflector lamps, defocussing the lamp image, cooling fans, infrared reflecting dichroic mirrors between the lamp and fiber bundle, and optically-tuned heat absorbers, such as seen in the applicant's U.S. Pat. No. 5,099,399.
The center of a fiber optic bundle at the bushing receiving projector light tends to slowly increase in temperature over extended time periods, as there is a limited means for dissipating heat from the central fibers. Since fiber life expectancy varies inversely with temperature, there is a need for an efficient way to dissipate heat from the center of the fiber bundle.
The basic purpose of the present invention is to provide a fiber optics bushing in which cooling air from outside a fan-cooled fiber optic projector is drawn through interstices between the fibers and across the illuminated face of the bundle of fibers to maintain a suitably low operating temperature